The development of drug combinations that act effectively against both wild-type and mutated, resistant forms of HIV-1 reverse transcriptase (RT) is a major goal in the management of HIV disease. Recent studies have shown that resistance to different nucleoside analog RT inhibitors (NRTI) can result in different amino acid substitutions in close proximity to the dNTP binding pocket of the enzyme. Some of these mutations have been shown to cause cross- or multiple drug-resistance among various members of this family of antiretrovirals. In contrast, certain combinations of amino acid substitutions can sometimes lead to increased drug susceptibility and may also result in the resensitization of formerly resistant viruses. A biochemical understanding of these complex viral phenotypes may be of major importance with respect to the development of novel chemotherapeutic strategies to interfere with the activity of mutated, drug-resistant enzymes.

One of the key mechanisms involved in explaining certain types of drug resistance, as well as resensitization, is pyrophosphorylysis, i.e. the reverse step of reverse transcription. All enzymatic reactions involve both forward and reverse steps, and RT is no exception to this rule. Simply put, the polymerization step in RT involves the addition of an additional base to a growing DNA chain; the reverse reaction involves the cleavage from that chain of the most recently added base alongside the production of pyrophosphate, i.e. PPi, as a by-product. Hence, the term pyrophosphorylysis.

An understanding of these considerations is important in regard to antiviral chemotherapy of HIV disease. For example, resistance to AZT is now known to result, at least in part, from an increase in pyrophosphorylysis that occurs when certain AZT resistance-conferring mutations are selected by this drug. This leads to the production of RT enzymes that contain altered amino acids at the mutated sites. The resultant increase in pyrophosphorylysis means that the most recently added nucleoside triphosphate base will be more likely to be cleaved away. If this base is AZT itself, then the consequence will be that AZT will no longer be stabley incorporated into the growing chain of viral DNA. As a consequence, chain termination will no longer take place in the usual fashion, since the AZT molecule will have been cleaved off. Thus, other bases can continue to be incorporated and viral replication can proceed.

In the case of resensitization to AZT and heightened sensitivity to other drugs, most evidence points to the M184V mutation in RT as being of special significance. In brief, this substitution is responsible for high-level resistance to 3TC and low-level resistance to both ddI and abacavir (ABC). One of the consequences of the M184V substitution in RT is a significant diminution in pyrophosphorylysis. This concept leads to a biochemical explanation of the resensitization that can occur on the part of 3TC-resistant viruses.

Studies have demonstrated viruses that contain both the M184V mutation in RT, as a consequence of resistance to 3TC, and AZT resistance-conferring mutations display renewed sensitivity to the latter drug. Simply put, the diminution in pyrophosphorylysis now means that a terminally-added AZT molecule will no longer be cleaved away and that AZT can continue to effect chain termination. This effect may only be transient, however, as other more complex patterns of RT mutations, that may emerge in the aftermath of combination antiretroviral therapy, may limit the benefits of this M184V-mediated diminished pyrophosphorylysis effect over time.

The diminution in pyrophosphorylysis conferred by M184V may also help to explain the fact that viruses containing the 184V substitution display heightened sensitivity to certain drugs, e.g. PMEA (adefovir) and PMPA (tenofovir). In brief, the active forms of these compounds seem to be incorporated into HIV DNA with greater efficiency by M184V-containing RT than by wild-type RT. This may be a consequence of diminished pyrophosphorylysis that results in less efficient removal of these chain-terminating drugs than would otherwise occur. Further studies of the biochemistry of RT and its multiple reactions will help to provide greater insight into the mechanisms involved in the resensitization and hypersensitization of this enzyme to different antiviral compounds.

Related HIVresistanceWeb articles:

Mechanisms of Antiretroviral Resistance.
(Mark A. Wainberg, M.D., Sep/Oct 1999)

Interactions Between Drug Resistance Mutations.
(Robert W. Shafer, M.D., May/June 1999)

Can the 184V Mutation Have Clinical Benefit?
(Mark A. Wainberg, M.D., May/June 1999)